Functional Electrical Stimulation

Discussion

In general, all studies reviewed involving FES produced beneficial results on muscle functions such as strength and endurance or muscle structure such as increased muscle size (i.e., reduced muscle atrophy). FES may have additional benefits over PES alone. In particular, the study by Baldi et al. (1998) should be highlighted as it was the only randomized, controlled trial (n=26) which compared FES (cycle ergometry exercise), PES (isometric exercise) and an untrained control group. These investigators assessed lean body mass in 3 distinct body areas (i.e. total body, lower limb, gluteal) as a marker of muscle atrophy in recently injured (approximately 10 weeks) individuals with motor complete SCI. Their results demonstrate that the FES-assisted cycling program is effective in reducing atrophy and resulted in relative increases in lean body mass in all areas after 3 and 6 months of participation. The PES-assisted isometric exercise group also reduced muscle atrophy but had intermediate results between FES and no treatment (their control group actually lost lean mass).

Effect Size Forest Plots of RCTs with Available Data

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Reversal of muscle atrophy also appears feasible in more longstanding complete or motor-complete SCI (i.e. > 2 years post-injury) as shown by increases in muscle cross-sectional area and the muscle/adipose tissue ratio using FES-cycling (Crameri et al., 2002; Scremin et al., 1999). In chronic SCI, fatigability is also a key issue due to changes in muscle fiber composition. Fornusek et al. (2013) proposed that lower FES cycling cadences may therefore be more beneficial as slower cycling could mitigate the onset of fatigue and allow greater muscle force production. Indeed, in a recent pilot study using each participant as their control, Fornusek et al (2013) provided preliminary evidence that a lower FES cycling cadence compared to a higher cadence (10 rpm vs. 50 rpm) could be more effective at improving muscle hypertrophy and isometric strength.

NMES may also be used to strengthen the atrophied muscles to some extent prior to FES (Kern et al., 2005; Kern et al., 2010a; Kern et al., 2010b) and in some cases, FES is not possible unless NMES is first used. Kern et al. (2005) used a progressive NMES – FES program for quadriceps building eventually leading to FES-assisted standing in people with longstanding complete cauda equina injuries (>1.2 years post-injury). These investigators demonstrated increases to the overall mean fiber diameter and the proportion of total cross-sectional area covered by muscle fibers with training as compared to an untrained group. Later studies showed that FES had similar results in a larger group of participants (Kern et al., 2010a, Kern et al., 2010b). However, the feasibility of providing life-long stimulation therapy to participants with denervation injuries is uncertain.

There was one null finding associated with muscle atrophy in that Gerrits et al. (2000) employed a relatively shorter program of 6 weeks of FES-assisted cycling exercise in people with longstanding motor complete SCI (> 1 year post-injury) and found no change in muscle size. These non-significant results might be due to the relative insensitivity of the measure of thigh circumference, especially with the short intervention period and the absence of a control group for comparison purposes.

In addition to improving muscle properties, FES-cycling can improve work output and endurance (Crameri et al., 2002; Gerrits et al., 2000). For example, Gerrits et al. (2000) used a short (6 weeks) pre-post trial of FES-assisted cycling intervention in people with motor complete SCI and found an increased resistance to fatigue in the quadriceps muscle and greater work output.

Some mechanistic investigations have been conducted which help to explain some of these adaptations to muscle morphology and function with ongoing electrical stimulation exercise programs. For example, using FES-assisted cycling, Koskinen et al. (2000) demonstrated an increase in total collagen content as well as up- and down-regulation of proteins consistent with muscle-building activity. Others have noted an adaptive response to FES-assisted cycling exercise that serves to limit or alter the shift in the oxidative properties or fibre type composition of muscles that typically occurs following SCI (Crameri et al., 2002).